Modelling impurity control in the JET pumped divertor

For the New Phase of JET; an axisymmetric pumped divertor is proposed to control the backflow of impurities form the target plates towards the main plasma. The idea is to entrench the impurities in a forced flow of plasma directed along the open magnetic field lines in the scrape-off layer and thereby avoid contamination of the bulk plasma. The creation of such a flow can be envisaged by the injection of deuterium gas or pellets into the scrape-off layer, for steady-state operation, a corresponding fraction of the flow has then to be pumped near the targets. Alternatively or concurrently, a fraction of the gas recycled at the targets could be recirculated to the scrape-off layer near the X-point. Following earlier work on modelling these two schemes, we report here on the results obtained for magnetic configurations corresponding to currents up to 6 MA, having improved the impurity model by developing a multistage model where radiation is computed consistently. We assess the level of flow which is required to entrench impurities near the target plates and the fraction of the deuterium flow at the targets which needs to be recycled near the X-point if this flow must be provided by recirculation. The flow is in the range of 5 x 1023 s-1, which for the conditions reached at the target plates corresponds to about 90% of the deuterium flow onto the plates. This estimate has been obtained for a configuration where the connection length could be near 10 m. In this case the flow requirement is eased since it scales roughly inversely linearly with the connection length. We show also that beryllium alone is unlikely to radiate the power required. (author) 7 refs., 4 figs